1. Field of the Invention
The present invention relates to a golf ball and more particularly to a dimple pattern of the golf ball.
2. Description of the Related Art
A golf ball has approximately 300 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight of the golf ball to accelerate transition of a turbulent flow at a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backwards so that a pressure resistance is reduced, resulting in an increase in a flight distance of the golf ball. Moreover, the acceleration of the transition of the turbulent flow increases a distance between upper and lower separating points of the golf ball which is caused by backspin. Consequently, lift acting on the golf ball is increased. Accordingly, a dimple pattern capable of easily accelerating the transition of the turbulent flow, that is, a dimple pattern capable of better disturbing an air stream is more excellent aerodynamically.
A regular polyhedron or a quasi-regular polyhedron (which will be hereinafter referred to as a xe2x80x9cpolyhedron) is often used for the dimple pattern. More specifically, a polyhedron inscribed on a sphere is supposed, and sides of the polyhedron are projected on a spherical surface by rays irradiated from the center of the sphere onto the spherical surface, thereby forming a comparting line. The spherical surface is comparted by the comparting line. Thus, the dimples are arranged. Examples of the polyhedron to be used include a regular hexahedron, a regular octahedron, a regular dodecahedron, a regular icosahedron, a cube-octahedron, an icosa-dodecahedron and the like.
The regular octahedron has been used for a general golf ball for a long time because dimples are systematically aligned finely. Twelve comparting lines obtained by projecting twelve sides of the regular octahedron form three great circles (which will be hereinafter referred to as a xe2x80x9ccomparting great circlexe2x80x9d). These comparting great circles are orthogonal to each other. The spherical surface is comparted into eight spherical regular triangles through the twelve comparting lines (that is, three comparting great circles). Dimples are arranged for each spherical regular triangle. Such a dimple pattern is referred to as a regular octahedron pattern. Usually, the dimple is provided on the inside of the spherical regular triangle and does not intersect the twelve comparting lines. Accordingly, the three comparting great circles do not intersect the dimple. Portions corresponding to the comparting great circles act as great circle paths where the dimple is not present. By the existence of the great circle path, there is an advantage that a directional alignment can easily be carried out before patting.
The golf ball is formed by upper and lower molds comprising semispherical cavities. A spew is generated in a portion (a so-called seam) corresponding to the parting lines of the upper and lower molds on the surface of the formed golf ball. The spew is ground and removed through a grindstone or the like. In an ordinary regular octahedron pattern, one of the three great circle paths is coincident with the seam. Consequently, the dimple is not present on the seam and the spew can easily be removed. Such a golf ball has been disclosed in Japanese Laid-Open Patent Publication No. Sho 60-11665 (1985/11665).
In the golf ball having the regular octahedron pattern, dimples are not present on the seam, so a dimple effect tends to be insufficient when the seam (to be the great circle path) is coincident with a portion in which a circumferential speed of backspin is the highest (which will be hereinafter referred to as the xe2x80x9chighest speed portionxe2x80x9d). As described above, the spew generated on the seam is removed by the grinding, so there is a possibility that the vicinity of the seam of the surface of the golf ball might be ground simultaneously during the removal and the dimples might be deformed, resulting in a reduction in the dimple effect. Furthermore, the dimple patterns on the right and left of the seam are identical or equivalent to each other and the identical or equivalent dimple patterns appear repetitively along the seam during the rotation of the golf ball. Therefore, the dimple effect tends to be insufficient when the seam is coincident with the highest speed portion. More specifically, in the golf ball having the regular octahedron pattern, the following three unfavorable conditions are satisfied on the seam:
(1) the seam is a great circle path having no dimple;
(2) dimples provided around the seam might be deformed by grinding; and
(3) a dimple pattern appearing along the seam by rotation is monotonous.
Moreover, the drawbacks (1) and (3) described above are caused when two other great circle paths, as well as the seam, are coincident with the highest speed portion.
Japanese Laid-Open Patent Publication No. Hei 11-70186 (1999/70186) has disclosed a golf ball having a regular octahedron pattern in which a dimple is provided on a comparting great circle. In the golf ball, the great circle path is not formed. Therefore, the drawback (1) can be eliminated. However, the drawbacks (2) and (3) are still caused on the seam. For two comparting great circles other than the seam, the drawback (3) is caused.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a golf ball comprising a dimple pattern to be a regular octahedron pattern and capable of preventing dimple effects from being reduced when a comparting great circle is coincident with the highest speed portion.
In order to achieve the above-mentioned object, the present invention provides a golf ball in which twelve sides of a regular octahedron inscribed on a surface of the golf ball are projected onto the surface so that the surface is comparted into eight spherical regular triangles through twelve comparting lines virtually formed and three great circles are formed, and a plurality of dimples are arranged on the spherical regular triangles and all the three great circles intersect the dimples,
wherein dimple patterns of four spherical regular triangles sharing each of six apexes of the regular octahedron positioned on the surface are not identical to each other,
dimple patterns of two spherical regular triangles sharing each of the six apexes of the regular octahedron positioned on the surface and opposed to each other are neither line symmetrical nor point symmetrical with each other, and
dimple patterns of two spherical regular triangles sharing each of the twelve comparting lines are neither line symmetrical nor point symmetrical with each other.
In the golf ball, as described below in detail, when the comparting great circle is coincident with the highest speed portion, the dimple patterns of right and left spherical regular triangles of the comparting great circle are neither identical nor equivalent to each other. Moreover, when the golf ball rotates, the spherical regular triangles having dimple patterns which are neither identical nor equivalent sequentially appear along the comparting great circle. Accordingly, the dimple patterns appearing through the rotation are not monotonous so that dimple effects can be enhanced when the comparting great circle is coincident with the highest speed portion. Consequently, the flight distance of the golf ball can be increased, and furthermore, flight performance can be prevented from being varied depending on a position of the highest speed portion.
It is preferable that all the twelve comparting lines should intersect the dimples. Consequently, the dimple effects can be more enhanced when the comparting great circle is coincident with the highest speed portion.
It is preferable that each of the eight spherical regular triangles should have an internal dimple pattern which is neither rotation symmetrical nor line symmetrical. Consequently, the dimple pattern in each spherical regular triangle approximates a disorder so that the dimple effects can be enhanced.
It is preferable that the number of dimples arranged in each of the eight spherical regular triangles should be 40 to 55. Consequently, excellent dimple effects can be produced and the flight performance of the golf ball can be enhanced.
In the eight spherical regular triangles, a difference between the number of dimples in the spherical regular triangle having the greatest number of dimples arranged therein and the number of dimples in the spherical regular triangle having the smallest number of dimples arranged therein is preferably four or less. Consequently, the aerodynamic symmetry of the golf ball can be enhanced.
It is preferable that there should be no dimple having a center thereof positioned on the comparting line. Consequently, the dimple intersecting the comparting line is unevenly present on the spherical regular triangles on both sides of the comparting line. Consequently, the dimple effects can be more enhanced.
The present invention is also suitable for a golf ball in which one of three comparting great circles is almost coincident with a seam. The seam has such a drawback that surrounding dimples might be deformed by grinding. However, the dimple patterns appearing through the rotation are not monotonous, so it is possible to prevent the dimple effects from being reduced when the seam is coincident with the highest speed portion.